Be Aware of a Potential Enemy in the Family Garden

Published: April 1, 2008

When putting in your garden this year, be aware of a pest on the increase in Missouri. This is the root knot nematode. This nematode is a microscopic worm that can infect vegetable roots causing serious symptoms and producing galls or “knots” on the roots. These galls impede the flow of water and nutrients to the plant, causing slow growth, yellowing, wilting easily in the heat of summer, stunting and reduced yields. This potential pest has a very wide host range (over 2000 hosts) and can reproduce on most lawn and garden plants.

Root knot nematode has been a scourge to gardeners in the southern regions of the state for many years, but it is just now beginning to be found in mid- and northern Missouri. A good deep soil freeze usually kills this nematode; however, our winters have been increasingly mild since the 1980’s. If this nematode gets introduced into your garden, you may have a long battle on your hands.

How does this nasty nematode get into your garden? Root knot can only enter a garden by soil or plant roots. Infected vegetable and flower transplants, garden tools (including tillers or tractors) with infested soil, muddy boots or vehicle tires and improperly or incompletely composted material are all possible sources. Thousands of root knot eggs or worms may be present in a tablespoon of soil. This means sharing garden transplants or renting, lending and borrowing garden equipment could be a source of nematode contamination. Usually store bought transplants grown in sterilized potting mix are free of deleterious nematodes.

Over the past couple of years, this nematode has shown up in a few samples from central Missouri submitted to the plant diagnostic clinic. Last summer a client who had a garden plot in one of the Columbia Community Gardens submitted a tobacco root from a plant that was not doing well. The sample had visible galls on the roots, and the soil where the plant was grown was found to have a population of 83,000 nematodes in 1/3 of a cup of soil. Unfortunately, this man had shared some transplants with friends, and later in the summer he found nematodes in their flower beds. A community garden may have a dozen or more different people with plots, often sharing equipment and a common compost pile. Nine community gardens were sampled and six of them tested positive for the root knot nematode. It is probable that the hundreds of gardeners with plots also had shared plants, and inadvertently spread the root knot nematode to other gardens.

The root knot nematode can be kept out of your home garden by practicing good garden sanitation. Always clean garden equipment if it has been used in another garden. Soap, water and a brush work well; if the soil is gone, the nematode should be too. Use only garden transplants from a reputable dealer. Do not add compost or soil to your garden unless you are sure that it is not infested. Garden roots infested with root knot should be destroyed and not composted. A properly managed compost pile should kill the root knot egg masses on the galls, but home compost piles are often neglected, and do not reach required high temperatures.

The root knot nematode can be controlled in a garden, but it is not very easy. First, it must be noted that there are no agricultural nematicides or other chemicals or fumigants that are labeled for home gardens. There are some organic substances developed from plants that have been used in gardens for the control of nematodes, but often with limited success. There have also been some microbial pathogens, such as bacteria and fungi, developed into commercial formulations against nematodes, again with limited success.

Root knot management strategies for a home garden

Normally, cold winters can prevent this pest from surviving and thriving. Winter cold can be enhanced by keeping a root knot infested garden clean of grass or other insulating debris. Winter tilling can also help facilitate the penetration of cold temperatures by turning over and loosening the soil.

There are a few non-host vegetables for root knot. They are from the cruciferous family—mustard greens, various cabbages, broccoli, cauliflower and brussels sprouts. Since the root knot nematode does not become active until the soil temperature reaches 64 degrees, early spring vegetables may also avoid damage. Likewise, planting summer vegetables as early as possible will give them a good start before the nematode becomes active.

Planting nematode resistant plants (look for VFN) is a good way of controlling root knot, especially if their numbers have not become excessive. There are many varieties of resistant tomatoes, but not many choices with other vegetables. After two years of resistant plants, the number of root knot nematodes should be low enough to plant a susceptible variety again. A season of susceptible plants, though, will allow the root knot population to again become high.

Keeping part of the garden fallow will reduce the numbers of root knot nematodes because they will not have a host on which to feed. The fallow section must be kept free of weeds to be effective, since most weeds are hosts. Covering the fallow section with black plastic may help.

Soil solarization can control root knot nematodes. Soil temperature that reaches 125 degrees for 30 minutes can kill root knot. The garden section to be treated should first be tilled and moistened. Then it should be covered with clear plastic (2-4 mil.) for the months of June-August.

There are some anti-nematode plants, such as French Marigolds, whose roots produce a substance that can be toxic to root knot. (Not all marigold varieties have this quality.) Some good varieties are: Bonita Mixed, Gypsy Sunshine, Scarlet Sophia, Single Gold, Petite Harmony, Petite Gold, Tangerine, Crackerjack and Flor de Muerto. Do not plant Tangerine Gem or hybrid marigolds since they may allow the nematode to increase. Planting marigolds around other susceptible garden vegetables is not effective; the nematodes will simply avoid the marigolds and feed on the vegetable roots.

With good planning and management skills the root knot nematode can be controlled by rotating sections of the garden between: resistant varieties, fallow, solarization, marigolds, and susceptible varieties. Sometimes, if there is the availability of land, it is easier to start a fresh garden spot, and plant the infested one to grass. Great care must be taken not to infect this new garden space.

The southern root knot nematode is still rare in mid and northern Missouri and with proper precaution it can be kept out of your family garden. Autumn is a good time to check that your garden is free of root knot. When cleaning up the garden, as you pull up the old plants, simply examine the roots for knots or galls. If the roots are bumpy with galls, you likely have root knot. A soil sample (include some root material) can be sent to the University of Missouri Extension Nematology Lab located at 23 Mumford Hall, University of Missouri, Columbia , MO 65211 to confirm that root knot is present. There is a $20 fee for this test. For additional information: visit the lab’s Website at http://soilplantlab.missouri.edu/nematode.